Valves that are utilized in nuclear power plants for main steam or main feed water isolation, or other critical applications (e.g., gas turbines, dampers, etc.), are generally opened and closed utilizing valve actuators. Valves utilized in such critical applications are normally used for emergency shut-off purposes. Thus, these valve actuators must be operable to close the valve in a rapid period of time, typically within about three seconds or so. Opening the valve is typically less of a concern, since the primary purpose of the valve is for an emergency shut-off situation. However, if the valve is used for an emergency open purpose, the valve must be operable to open in a rapid period of time. Typical actuators utilized in such critical and non-critical applications include, but are not limited to, mechanical, electromechanical, hydraulic, pneumatic, and hydro-pneumatic powered actuators.
Various regulations govern the operational requirements for valves utilized in nuclear power plants. One such operational requirement is directed toward characterizing the flow of system media through the valve during the closing stroke of the valve in a predetermined manner. Typically, the system media flow through the valve has been characterized by an exponentially decreasing flow curve as the valve progresses through its closing stroke. However, recent requirements governing nuclear power plant use have called for a constant, or straight line, decreasing system media flow curve during the closing stroke of the valve. Thus, the valve actuators utilized in nuclear power plants will need to operate to close the valve in such a manner that the system media flow through the valve during its closing stroke is characterized by a constant, straight line decreasing curve.
Additionally, other applications may call for other desired system media flow curves during the valve closing stroke. Further yet, certain applications may call for desired system media flow characteristics during the valve opening stroke. While servo-electronics and fine electrical control have been utilized to characterize system media flow through valves in desired manners, when valves are used in applications such as for nuclear applications in containment or other harsh environments (e.g., excessive vibration, high temperatures, radiation, etc.), the use of servo-electronics and/or fine electrical systems presents a problem, in that the servo-electronics and/or the fine electronics often cannot survive in the environment in which the valves are used. For critical applications where proper operation of the valve is essential (e.g., for safety concerns), the failure of servo-electronics and/or fine electrical systems to operate properly in opening and/or closing the valve can have catastrophic results.
The present invention is directed toward overcoming one or more of the above-mentioned problems.